Increasing power and power densities in new, high complexity semiconductor devices are creating heat removal problems which are taxing current packaging capabilities. Increased power produces higher junction temperatures which lead to potentially faster degradation of both device reliability and system performance. Current user trends towards higher ambient temperatures combined with severe limitations on external cooling, and potting of packages for applications such as those associated with automotive products create additional factors which require consideration. With cost pressures compounding the problem, low cost packages require upgraded thermal performance to meet increased performance demands.
Dual in-line packages (DIP's) have been, and still are, the "workhorse" of integrated circuit (IC) packaging. Plastic DIP's dominate most low reliability applications. The ruggedness, low cost, and handling capabilities of plastic DIP's are attractive characteristics for high volume usage. Their increased use in hostile environments, combined with increased device and mounting surface temperatures, has resulted in extensive attempts to improve the design and materials of the plastic package. Unfortunately, constraints on materials selection, package design, and heatsinking make improvements in heat removal more difficult to obtain than from other discrete and ceramic packages. The effect and improvement in packaged devices thermal characteristics is reviewed in an article entitled Thermal Characteristics of 16- and 40-Pin Plastic DIP's by Andrews et al. published in the IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS AND MANUFACTURING TECHNOLOGY, VOL. CHMT-1 NO. 4, DECEMBER, 1981.
A plastic DIP is a plastic molded package in which an electronic device, supported on a lead frame, is molded into an encapsulating plastic. This plastic package has several reliability problems. Firstly, failures in the plastic to metal bond provide an avenue through which moisture and other contaminants can contact the electronic device and cause corrosion failures. This problem has been documented in an article entitled Factors Governing Aluminum Interconnection Corrosion in Plastic Encapsulated Microelectronic Devices by Neighbour and White, published in MICROELECTRONICS AND RELIABILITY, Pergamon Press, Great Britain, Vol. 16, 1977, pages 161-164.
Even when corrosion failures do not occur, diffusion of environmental contaminants through the plastic or through the gradual degradation of the plastic may lead to malfunction of the encapsulated electronics. These sources of malfunction have been examined in an article entitled Factors Contributing to the Corrosion of the Aluminum Metal on Semiconductor Devices Packaged in Plastics by Olberg and Bozarth, published in MICROELECTRONICS AND RELIABILITY, Pergamon Press, Great Britain, Vol. 15, 1976, pages 601-611.
The prior art recognizes the combination of antioxidants with polyolefins to provide a heat stabilizer against formation of free radicals during the manufacturing process. An example is disclosed in U.S. Pat. No. 4,213,892. The antioxidant functions by controlling chain scission. However, it is not thought that the prior art discloses a combination antioxidant and metal deactivator (described below) being grafted onto a polyolefin.
Attempts have heretofore been made to improve adhesion of a polymer to metals by adding metal deactivators to the polymer as illustrated in U.S. Pat. Nos. 3,907,925, 3,994,987, 4,049,875, 4,077,948 and 4,306,930. The metal deactivators are provided to prevent the degradation of the polymer by metal ions. These metals, such as titanium or magnesium, may be present as chlorides. The metals tend to degrade the polymer over time. In addition, when the polymer is adhered to the surface of copper, the copper being a good catalyst for most polymer degradation, provides copper ions which tend to migrate into the bulk of the polymer from the interface between the copper and the polymer. The metal deactivators chelate and capture the ions going into the polymer from the interface.
A well-known compound for preventing corrosion is maleic acid or maleic anhydride. The grafting of maleic acid (Jpn. Kokai Tokyo Koho No. 58,183,733) or maleic anhydride (Jpn. Kokai Tokyo No. 58,166,040) onto polyolefins for improved polymeric adhesion to metal surfaces is well known in the art. However, these grafted polymers show instability toward oxidation when adhered to a metallic surface such as copper, since copper acts as an efficient catalyst for oxidation in polyolefins.
To the best of our knowledge, the grafting of a dual functional metal deactivator and antioxidant molecule onto a polyolefin is unknown in the art. Such a grafted polymer would be expected to exhibit both good metal adhesion, due to the metal chelating functionality, and good oxidative stability, due to the antioxidant and metal deactivating functionality.